One of the most difficult tasks facing radiographers using conventional radiographic techniques (e.g., x-ray film) is producing a diagnostic image of uniform optical density when examining a body part that varies greatly in thickness or tissue composition. Conventional compensation techniques for density variation typically include a compensating filter. Compensating filters can be fabricated for many procedures, and therefore come in various sizes and shapes. They are usually constructed of aluminum or plastics.
One commonly used filter is a wedge filter. The wedge filter is used when exposing a body part, such as the human foot, to x-rays. For example, during examination of the foot, the wedge is positioned with its thick portion shadowing the toes and the thin portion toward the heel. The thick portion of the wedge absorbs excess radiation, thereby preventing overexposure of the toes. The thin portion of the wedge allows more radiation to penetrate the heel, thereby preventing underexposure of the heel. The overall benefit of using the wedge is to provide an image with uniform optical density.
Alternatively, wedge filters may be used to calibrate x-ray machines and evaluate radiography procedures. For example, when an object of varying thickness is x-rayed, a "step" wedge of the same material incorporating the same thickness variations can be used to determine sensitivity levels for each thickness. Step wedges comprise a continuous series of steps which can be manufactured to a customer's specifications for any height or width, step dimensions, total number of steps, and total height of wedge. By placing, for example, a penetrameter on each step of the wedge and exposing the step wedge to x-rays, the sensitivity levels for each thickness of an object can be determined.
Other types of compensating filters are commonly used with x-ray procedures and/or systems. These include "trough" filters for examining the chest, "bow-tie" filters for use with CT to compensate for the shape of the head or body, and "conic" filters (e.g., concave, convex) for use in digital fluoroscopy, where the image receptor and the image-intensifier tube are round.
Currently, digital radiography systems are beginning to replace conventional x-ray systems. Digital radiography systems provide high quality radiographs by capturing x-ray images with a sensor plate having a matrix or array of silicon detectors. The x-ray images can be transmitted to a diagnostic viewer or any other output device, or to any other location via, for example, an Ethernet interface.
The sensor plate provides several advantages over conventional x-ray film. For example, unlike conventional x-ray film, digital images can be previewed within a few seconds of x-ray exposure. Moreover, the sensor plate used in digital systems can capture most patient imaging areas with high resolution (e.g., 160.times.160 microns pixel size, with 4096 gray scale (12 bit) contrast). The sensor plate also covers a larger dynamic range than conventional x-ray film.
Like conventional x-ray film, digital radiography systems require exposure compensation for certain procedures. It is desirable therefore to have a system and method for compensating exposure deficiencies in a digital radiography system. Such a system and method should be easily integrated with existing digital radiography systems and provide simple, low cost exposure compensation without using conventional compensation filters.